President Barack Obama sent Congress a request Wednesday for authorization to use military force in the campaign against the Islamic State group, urging lawmakers to "show the world we are united in our resolve to counter the threat."

In a letter to Congress, the president said he is committed to working with Congress on bipartisan authorization for the use of military force.

The proposed authorization for the use of military force (AUMF) would limit operations against the militants to three years and bar U.S. troops from what it terms "enduring offensive ground combat."

Obama's draft proposal said large-scale ground combat operations, similar to those in Iraq and Afghanistan, should be left to local forces instead of the U.S. military.

The proposal also would repeal the 2002 measure that authorized the Iraq war but keep a 2001 authorization, passed shortly after the September 11 attacks, for a campaign against al-Qaida and its affiliates.

"This is a difficult mission, and it will remain difficult for some time," the president said in nationally televised remarks. "But our coalition is on the offensive. ISIL is on the defensive, and ISIL is going to lose."

The president says the authorization would give him the flexibility he needs to go after the militants.

“For example, if we had actionable intelligence about a gathering of ISIL leaders, and our partners didn’t have the capacity to get them, I would be prepared to order our Special Forces to take action, because I will not allow these terrorists to have a safe haven,” he said.

Congressional approval

Obama's proposal must be approved by both the U.S. Senate and House of Representatives.

The proposal is expected to provoke strong debate between Democrats, who are generally wary of another Middle East war, and Republicans, many of whom have been pushing for stronger measures against the militant fighters.

Republican Senate Majority Leader Mitch McConnell said Wednesday senators will "listen closely to the advice of military commanders as they consider the best strategy for defeating ISIL," referring to an acronym for the Islamic State group.

Republican House Speaker John Boehner said, "While I believe an AUMF against ISIL is important, I have concerns that the president’s request does not meet this standard. ... Now we will begin hearings and rigorous oversight so lawmakers and the public can provide their input."

Democratic Senator Tim Kaine of Virginia, a close ally of Obama's and who has been seeking the military authorization for months, said Wednesday, "Finally, we are at the point where Congress is going to take seriously its most solemn obligation.”

Several Democrats voiced concerns, saying the president’s resolution limiting operations against Islamist militants to three years is too vague.

“The language pertaining to ground troops ... is very broad, very ambiguous. None of us really knows what 'enduring offensive combat operations' means. And deliberately, I think, [it is] drafted to be ambiguous," Schiff said.

Democratic House Minority leader Nancy Pelosi spoke for many who are wary of the U.S. getting involved in another protracted ground war in the Middle East, saying the American people have no appetite for what she called U.S. “boots on the ground.”

Some analysts said the president may face more opposition from his fellow Democrats than from Republicans, who now control both the House and the Senate.

White House spokesman Josh Earnest said Tuesday that while Obama believes he has the authority to order military action against the Islamist group, the new authorization would represent a “powerful symbol” of unity to degrade and destroy the group.

"The president does believe that this message is even more powerful if it has bipartisan support," said Earnest.

US citizens involved

Meanwhile, U.S. lawmakers on Wednesday are hearing testimony from counter-terrorism experts that more than 20,000 volunteers from more than 90 countries are flowing into Iraq and Syria to join Islamic State fighters.

The rate of such volunteer fighters is unprecedented and exceeds that of those who went to fight in Afghanistan, Pakistan, Iraq, Yemen or Somalia in the past 20 years.

One of those testifying before Congress is Rick Brennan, a RAND Corporation senior political analyst and career Army officer.

Brennan said this movement represents the generational challenge of our time. He also said radical Islamist groups have multiplied in recent decades.

"Going back to 1988, there were only three of these types of groups that existed in the world. By 2001, that number had expanded to 20 and the growth of those groups continued at a very steady pace until 2010," he said.

"But, by 2013, the largest as a result of the turbulence created by the Arab Spring, the number jumped from 32 in 2010 to 51 in 2013," Brennan said.

Individual foreign fighters have jumped from up to 22,000 in 2001 to as many as 90,000 in 2013, he said, and the success of the Islamic State group in 2014 has caused those numbers to skyrocket.

Victor Beattie contributed to this report from Washington and Luis Ramirez contributed from the White House. Some material for this report came from Reuters and AP.